that some peptide substitutions affect or V„ usage 
as well, suggesting that V-region structures can 
differ significantly from one another and limit the 
peptide contacts that can be made. Thus, although 
these results now constitute the most convincing 
demonstration of the original CDR3 ^ peptide con- 
tact model, there are clearly other aspects to the 
problem that require careful study. (A grant from 
the National Institutes of Health provided support 
for the project described above.) 
In addition, the group has made a series of mu- 
tants that are predicted to point "up" toward the 
TCR on both a helices of E*'. Most of these mutants 
bind peptide normally, but several interfere with 
TCR recognition and/or superantigen reactivity. 
Work in progress may reveal which of these mutated 
residues interacts with what parts of the TCR. 
The laboratory has also recently derived monoclo- 
nal antibodies specific for a particular peptide-MHC 
complex. Studying how these antibodies bind may 
give useful insights into the way B and T cell antigen 
receptors solve the same "problem." 
Peptide-MHC Interactions 
Dr. Davis and his colleagues have successfully 
made GPI-linked chimeras of an TCR and of the 
class II MHC molecules I-E''. In particular, an I E'' 
chimera on the surface of fibroblast cells can pres- 
ent peptides to most T cells (albeit at a reduced 
efficiency) but cannot present processed antigen. 
Cleaved soluble protein can present peptides effi- 
ciently and specifically to T cells when immobilized 
on plates, and the efficiency of antigen-MHC com- 
plex formation and stimulation of T cells is signifi- 
cantly greater at pH 5.0 than at neutral pH. This is 
consistent with the biology of class II MHC mole- 
cules as they cycle through endosomal compart- 
ments, some of which approach this pH in acidity. 
The laboratory has measured the kinetic parame- 
ters of the peptide presentation effect and find that 
for two different peptides, the on-rate increases 50- 
fold at pH 5.0 (vs. 7.3). In the case of the cy- 
tochrome peptide, there is only a modest change in 
the off-rate. In contrast, a second, more weakly anti- 
genic peptide has a similar increase in its on-rate (at 
pH 5.0) but is 1 00-fold weaker than the cytochrome 
peptide, which can efficiently displace it. This pro- 
vides a model system for how competition between 
peptide fragments may occur in the cell and sug- 
gests that "good" peptide antigens may be strongly 
selected for their ability to stay bound to an MHC 
molecule. In addition, the generation of monoclo- 
nal antibodies specific to a cytochrome peptide-E'' 
complex (using soluble material as the immuno- 
gen) provides a useful reagent for determining 
where in the cell substructure peptides first bind to 
class II MHC. 
Antigen-MHC and TCR Structure 
Dr. Davis's group, in collaboration with Dr. Paul 
Driscoll (Oxford University) and Drs. Ettore Appella 
and Jim Omishinki (National Institutes of Health), is 
using two-dimensional nuclear magnetic resonance 
(NMR) spectroscopy to try to visualize the solution 
structure of a peptide in a class II MHC molecule. 
They have successfully prepared large quantities 
(20 mg) of soluble I E'', containing both '^C alanine- 
labeled cytochrome peptides, and have obtained in- 
formation about its conformation and major areas in 
contact within the MHC. They are now analyzing a 
fully '^C- and ^^N-substituted peptide to get a more 
detailed picture of how the peptide binds to the E*' 
molecule. This information will also aid computer 
models of E'' based on class I structures and the pep- 
tide made in collaboration with Drs. Sebastian Don- 
iach and Michael Levitt (Stanford University). 
X-ray crystal lographic work will continue, espe- 
cially on the promising soluble TCR and peptide- 
MHC complex crystals obtained (with materials 
supplied by Dr. Davis) in Dr. Pamela Bjorkman's lab- 
oratory (HHMI, California Institute of Technology) . 
This latter has the advantage over the NMR study in 
potentially providing information about the entire 
MHC molecule. Ultimately the goal is to cocrys- 
tallize the soluble TCR with the antigen-MHC 
complex. 
Affinity of T Cell Recognition 
Dr. Peter Parham once remarked that "the T cell 
receptor is the only receptor that has never been 
shown to have received anything." Regardless, Dr. 
Davis and his colleagues have been working for 
many years to try to analyze the kinetic parameters 
of TCR recognition, making use of the group's solu- 
ble molecules. Success has finally come through use 
of soluble peptide-MHC complexes to compete 
with a labeled anti-TCR antibody fragment. In the 
several cases studied, the group derived values of 
4-6 X 10-5 ^ 1,000- to 10,000-fold weaker than 
for antibodies to comparable-sized protein ligands. 
These measurements have recently been confirmed 
using soluble TCR to compete with a peptide-MHC 
complex-specific antibody for its ligand. 
These values are consistent with the "scanning" 
nature of T cell recognition and indicate that TCR 
binding to peptide-MHC ligands is so weak energeti- 
cally that other, antigen-independent receptor- 
ligand systems must govern the initial stages of T 
cell contact with antigen-presenting or target cells. 
The best candidates for such regulators of T cell in- 
IMMUNOLOGY 323 
